Corrosion sensors

ABSTRACT

Corrosion sensor apparatus for detection of contamination affecting metal based components and devices. For example, an apparatus includes: a set of corrosion sensor elements, wherein a width of a first corrosion sensor element is different than a width of a second corrosion sensor element, wherein each corrosion sensor element is susceptible to corrosion caused by an operating environment of the corrosion sensor elements; and a set of reference elements wherein a width of a first reference sensor element is substantially equal to the width of the first corrosion sensor element and a width of a second reference sensor element is substantially equal to the width of the second corrosion sensor element, wherein each reference sensor element is not substantially susceptible to corrosion caused by the operating environment of the corrosion sensor elements.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a divisional of U.S. patent application Ser.No. 12/854,416, filed on Aug. 11, 2010, the disclosure of which isincorporated herein by reference in its entirety.

FIELD

The present application relates to detection of gaseous contaminationaffecting metal-based components, artifacts and devices such aselectrical and electronic components and devices, and more particularlyto corrosion sensors providing such detection.

BACKGROUND

Many facilities and buildings like museums, archives, clean rooms, datacenters, or pharmaceutical labs require tight control on air quality(dust and gaseous contamination). Extensive effort is devoted to reduceair contamination and maintain safe operating conditions. Atmosphericcontamination is known to exhibit geographical and temporal variationssince it is closely tied to local human pollution activities such aspower production, agriculture, transport, etc. To overcome atmosphericpollution in the above mentioned facilities and the like, air filtrationand air recirculation/conditioning is commonly employed.

Data centers are known to be one of the most energy intensive types offacilities that require controlled atmosphere. A data center is afacility used to host computing systems (e.g., servers) and associatedcomponents, such as telecommunications and storage systems. A datacenter also generally includes redundant or backup power supplies,redundant data communications connections, environmental controls (e.g.,air conditioning, fire suppression) and security devices. Oneenvironmental control approach provides for obtaining and utilizingoutside air (i.e., outdoor air) to cool and to control the temperatureof hardware associated with the computing systems of a data center.

However, the outside air cooling approach increases concerns of thepresence of gaseous contaminants in data centers caused by theintroduction of outside air. For example, pollution in the outside airintroduces gaseous contaminants that attack (corrode) copper lines oncircuit boards, individual components, or silver based soldering joints.Sulfur-bearing gases such as SO₂ and H₂S, nitrogen-based gases such asNO and NO₂, or reaction with toxic gases such as Cl₂ and Br can resultin corrosion products being formed. The concentration of these gases arein the range of parts per billion (ppb), however, large temporalvariations over the same geographical area can be commonly encountereddue to weather, geography and economic activities. The indoor gaseouspollutants can be related to the outdoor gaseous concentration, as morepolluted regions will have a higher indoor contamination. Thus, by wayof example, for cooper lines on a circuit board, copper-sulfide isformed which creeps over the circuit board and short circuits closelyspaced lines. For silver based soldering joints, silver-sulfide isformed which eats away at the silver, causing open circuits.Furthermore, airborne dust is also known to chemically corrode printedcircuit board components.

A recent American Society of Heating, Refrigerating, andAir-Conditioning Engineers (ASHRAE) recommendation entitled “Gaseous andParticulate Contamination Guidelines for Data Centers,” Aug. 2009, thedisclosure of which is incorporated by reference herein in its entirety,set an upper limit of an acceptable corrosion level in data centers at30 nanometers per month (nm/month) for both copper and silver.

SUMMARY

Illustrative embodiments of the invention provide methods and apparatusfor detection of contamination affecting metal-based components,artifacts and devices such as electrical and electronic components anddevices, and more particularly to corrosion sensors providing suchdetection.

For example, in a first embodiment of the invention, an apparatuscomprises: a set of corrosion sensor elements comprising a firstcorrosion sensor element and at least a second corrosion element, eachcorrosion sensor element being coupled between a source and a monitor,wherein a width of the first corrosion sensor element is different thana width of the second corrosion sensor element, wherein each corrosionsensor element is susceptible to corrosion caused by an operatingenvironment of the corrosion sensor elements; and a set of referenceelements comprising a first reference sensor element and at least asecond reference sensor element, each reference sensor element beingcoupled between the source and the monitor, wherein a width of the firstreference sensor element is substantially equal to the width of thefirst corrosion sensor element and a width of the second referencesensor element is substantially equal to the width of the secondcorrosion sensor element, wherein each reference sensor element is notsubstantially susceptible to corrosion caused by the operatingenvironment of the corrosion sensor elements.

In one example, a signal generated from the first corrosion sensorelement in response to the source is received by the monitor and iscompared to a signal generated from the first reference sensor elementin response to the source and received by the monitor, wherein theresult of the comparison is indicative of an amount of corrosionexperienced by the first corrosion sensor element for a first period oftime. Further, a signal generated from the second corrosion sensorelement in response to the source is received by the monitor and iscompared to a signal generated from the second reference sensor elementin response to the source and received by the monitor, wherein theresult of the comparison is indicative of an amount of corrosionexperienced by the second corrosion sensor element for a second periodof time.

Furthermore, each of the corrosion sensor elements is preferably formedsuch that the corrosion sensor element corrodes in a single spatialdimension (e.g., width). The single spatial dimension corrosionadvantageously yields a linear response from the apparatus.

In a second embodiment of the invention, an apparatus comprises: a firstsensor element; and at least a second sensor element; wherein the firstsensor element and the at least a second sensor element are susceptibleto corrosion caused by an operating environment in which they reside,and wherein the at least a second sensor element has a corrodiblespatial dimension that is progressively larger than a corrodible spatialdimension of the first sensor element such that the at least a secondsensor element provides for corrosion monitoring for a time period thatis progressively larger than a period of time for corrosion monitoringassociated with the first sensor element.

In a third embodiment of the invention, an apparatus comprises: a firstcorrosion sensor; a second corrosion sensor; and at least a thirdcorrosion sensor; wherein the first, second, and third corrosion sensorsare configured in a bridge arrangement such that when none of the first,second, and third corrosion sensors experiences corrosion, the bridgearrangement is in a balanced condition, and when at least one of thefirst, second, and third corrosion sensors experiences corrosion, thebridge arrangement is in a unbalanced condition. Each of the first,second and third corrosion sensors may comprise a first pair of thinfilm sensor elements that are exposed to corrosion and a second pair ofthin film sensor elements that are not exposed to corrosion such thatthe amount of unbalance is directly related to corrosion of a width ofthe exposed sensor elements and a corrosion rate associated with theexposed sensor elements. Widths of the thin film sensor elements foreach of the first, second and third corrosion sensors may progressivelyincrease. The progressively increasing widths of the sensor elementsprovides for corrosion monitoring over progressively increasing timeperiods.

In a fourth embodiment of the invention, a method for fabricating acorrosion sensor device comprises the following steps. On a substrate, aset of corrosion sensor elements is formed comprising a first corrosionsensor element and at least a second corrosion element, each corrosionsensor element being coupled between a source and a monitor, wherein awidth of the first corrosion sensor element is different than a width ofthe second corrosion sensor element, wherein each corrosion sensorelement is susceptible to corrosion caused by an operating environmentof the corrosion sensor elements. Further, on the substrate, a set ofreference elements is formed comprising a first reference sensor elementand at least a second reference sensor element, each reference sensorelement being coupled between the source and the monitor, wherein awidth of the first reference sensor element is substantially equal tothe width of the first corrosion sensor element and a width of thesecond reference sensor element is substantially equal to the width ofthe second corrosion sensor element, wherein each reference sensorelement is not substantially susceptible to corrosion caused by theoperating environment of the corrosion sensor elements.

Advantageously, illustrative embodiments of the invention provide aresistive sensor with a linear response to corrosion that can bedeployed for real time measurement of corrosion with high sensitivityand also over an extended period of time. These sensors could bedeployed for real-time monitoring of the corrosion caused bycontaminations attributed to both inside and outside environment andthey could be deployed as integral parts of the air cooling process.Such inventive corrosion sensors overcome sensitivity problems withexisting sensor designs, as well as non-linearity problems whereby theresponse of existing sensors is non-linear as corrosion proceeds bothfrom top to bottom and from the sides of the metal film, thus makingdata interpretation difficult and inaccurate.

These and other objects, features, and advantages of the presentinvention will become apparent from the following detailed descriptionof illustrative embodiments thereof, which is to be read in connectionwith the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a top view of a corrosion sensor, according to anembodiment of the invention.

FIG. 2A illustrates a cross section view of a corrosion sensitiveresistor arm of a corrosion sensor before corrosion, according to anembodiment of the invention.

FIG. 2B illustrates a cross section view of a corrosion sensitiveresistor arm of a corrosion sensor during or after corrosion, accordingto an embodiment of the invention.

FIG. 3 illustrates a top view of a measurement setup for a corrosionsensor, according to an embodiment of the invention.

FIG. 4 illustrates a top view of a bridge detection setup for acorrosion sensor, according to an embodiment of the invention.

FIG. 5 illustrates detection sensitivity for sensor elements, accordingto an embodiment of the invention.

DETAILED DESCRIPTION

Illustrative embodiments of the invention may be described herein in thecontext of detecting atmospheric contaminants in data centerenvironments. However, it is to be understood that the techniques of theinvention are not limited to use in data centers but are more broadlyapplicable to detection of corrosive contaminants affecting anyelectrical and electronic components and devices. Furthermore, suchinventive corrosion sensors can be used to monitor corrosion of anysuitable types of metal-based components, artifacts and devices that aresusceptible to corrosion.

As will be explained in detail herein, illustrative embodiments of theinvention provide a metal film resistor based corrosion sensor with alinear response that is capable of sensing corrosion rates down to asensitivity of approximately zero nm/month. The sensors are optimized towork on a relatively low corrosion rate with corrosion product formed ata rate of about 30 nanometers/month or lower. A “thin metal film” or“metal film” is an example of a “sensor element” that may be employed bythe corrosion sensor of the invention. The metal film (e.g., silverfilm, copper film, etc.) is also referred to herein as a “metal wire”and may alternatively be considered a “nanowire.” Metal films arecurrent conducing elements with a resistance characteristic that isdependent on their spatial dimensions. It is to be appreciated that thespatial dimensions of the sensor elements used in accordance with theinvention are dependent on the type of corrosion being detected and theduration of the time period intended for detection. Thus, corrosionsensor principles of the invention are not intended to be limited to anyspecific spatial dimensions or specific range of spatial dimensions.

Rather, the sensor according to illustrative embodiments has multiplecorrosion sensitive resistor arms with progressively increasing widthsthat can be used to sense the corrosive environment for an extendedperiod of time with the same high sensitivity by selectively changingthe sensing arm of the sensor being monitored. The metal film (wire)based resistors change their resistance as corrosion proceeds from thesides of the film or wire, while the top of the film or wire isprotected with a corrosion resistive material, as will be explained indetail below. This ensures a linear response for the inventive sensor.

Highly sensitive corrosion sensors, such as those described herein inaccordance with principles of the invention, are advantageous for use ingaseous contamination monitoring in museums, hospitals, and data centersso as to mitigate and control the amount of outside air allowed in suchfacilities. That is, when a certain level of corrosion is detected viathe sensor response, the cooling environment of the facility can becorrespondingly regulated to reduce the cause of the corrosion level,i.e., gaseous contaminants in the outside air supply used in thefacility cooling process. This may be accomplished by filteringtechniques, or cutting back or even eliminating outside air for use incooling components of the facility.

FIG. 1 illustrates a top view of a multi-metal film corrosion sensor,according to an embodiment of the invention. As shown, corrosion sensor100 comprises a set of multiple corrosion sensor elements 102-1 through102-4, with progressively increasing widths. The corrosion sensorelements 102 are in the form of metal films, although other suitablesensor elements may be employed. The corrosion sensor 100 also comprisesa set of reference sensor elements 104-1 through 104-4, withprogressively increasing widths. The reference sensor elements 104 arealso in the form of metal films, although other suitable sensor elementsmay be employed. Note that the width of the corrosion sensor element102-1 is equal to (or approximately or substantially equal to) the widthof the reference sensor element 104-1. The same is true for sensorelements 102-2 and 104-2, for sensor elements 102-3 and 104-3, and forsensor elements 102-4 and 104-4. The rationale for this will beexplained below.

As further shown in FIG. 1, the corrosion sensor 100 also comprises aconnecting bus 106. The connecting bus 106 is composed of the same metal(electrical conducting material) as the variable width sensor elements(102 and 104). The connecting bus 106, like the reference sensorelements but unlike the corrosion sensor elements, has a conductivecoating to protect it from corrosion.

While not visible in FIG. 1 (but which is visible in FIG. 2 as 206), thecorrosion sensor 100 also comprises a substrate. The substrate may beany substantially non-conducting material, by way of example only, glassor glass-based, or silicon or silicon-based. The substrate maintains thesensor elements 102 and 104, the connecting bus 106, and contact pads108 and 110 in place so that they remain electrically connected, as willbe explained. In one embodiment, the various components of the corrosionsensor 100 are formed on top of the substrate via well-known electronbeam lithography fabrication processes.

Contact pads 108 are electrically conductive pads to which the sensorelements 102 and 104 are respectively connected. That is, each sensorelement 102 and 104 has one of its ends connected to its own dedicatedcontact pad 108. Contact pads 108 are then connected to a current orvoltage reading device (i.e., monitor) so that a current signal orvoltage signal can be received by the monitor. As will be explainedbelow, these signals are used to detect the level of corrosion beingexperienced by the corrosion sensor 100.

Contact pads 110 are also electrically conductive pads which connect tothe connecting bus 106. Thereby, the sensor elements 102 and 104 arerespectively connected to the contact pads 110. In this case, eachsensor element 102 and 104 has the other one of its ends connected toone contact pad 110 (via connecting bus 106) that is itself connected toa voltage supply source V. The other contact pad 110 is connected to aground potential. Thus, one contact pad 110 is grounded while thecurrent from contact pad 108 is going to a detection circuit that can bea trans-impedance amplifier or voltage detector.

Also shown in FIG. 1, the corrosion sensor 100 comprises anencapsulation layer 112. Encapsulation layer 112 encapsulates thereference sensor elements 104-1 through 104-4 and the connecting bus 106to protect them from the corrosive atmospheric environment to which thecorrosion sensor elements 102-1 through 102-4 are exposed. Theencapsulation layer 112 may be formed from materials such as thickdielectric film that is inert to corrosion, e.g., SiO₂, Si₃N₄, ororganic coatings like polystyrene or other corrosion resistantmaterials.

In accordance with this illustrative embodiment, the corrosion sensor100 operates such that the resistance of each corrosion sensor element102, in this case, each metal film (wire) 102, changes due to a reducedconduction path as the width of the metal film is converted from a metalto a nonconductive oxide due to corrosion. As the corrosive gases in theatmospheric environment, in which the sensor 100 operates, attack themetal of the metal film and transform it to a nonconductive oxide, thewidth is reduced thus increasing the resistance of the metal film.

The sensor elements 102 and 104 of the corrosion sensor 100 arepreferably fabricated by electron beam lithography. Further, theconnecting bus 106 may be formed that has arms (sensor elements)extending along the sides, such as is illustrated in FIG. 1. The sensorelements have variable (progressively increasing) widths and the widthof each sensor element determines the lifetime of the sensor element.

As an example, as shown in FIG. 1, the progressively-increasing widthsof the sensor elements 102 and 104 can be fabricated to be about 60 nm(sensor elements 102-1 and 104-1), about 120 nm (sensor elements 102-2and 104-2), about 180 nm (sensor elements 102-3 and 104-3), and about360 nm (sensor elements 102-4 and 104-4) wide. For a corrosion rate of30 nm/month (recall the ASHRAE recommendation mentioned above), the 60nm wide sensor element (102-1) could be used for corrosion monitoringfor a time duration of one month, while the 120 nm wide sensor element(102-2) would be usable for a two month period, and the 180 nm widesensor element (102-3) would be usable for a three month period.Although not expressly illustrated in FIG. 1, there could be 240 nm and360 nm wide corrosion sensor elements for use for time durations of fourand five months, respectively. The 360 nm wide sensor element (102-4)would be usable for a six month period. Of course, shorter/longerindividual time intervals can be achieved simply bydecreasing/increasing the progressively-increasing widths. Likewise,less/more time durations can be achieved simply by subtracting/addingcorrosion sensitive resistor arms.

It is to be appreciated that the reduced width assures that the changesassociated with a relatively small corrosion rate can be easily detectedas the change in width is significant compared with the total width. Thewidth of the sensor elements can be fabricated according to the expectedcorrosion rate in the environment and, in this embodiment, would betwice as large as the expected corrosion thickness due to corrosionproceeding from both sides.

This operation is illustrated in the context of FIGS. 2A and 2B, whereFIG. 2A illustrates a cross section view of a corrosion sensitiveresistor arm of a corrosion sensor before corrosion, and FIG. 2Billustrates a cross section view of a corrosion sensitive resistor armof a corrosion sensor during or after corrosion. The particularcorrosion sensitive resistor arm shown in FIGS. 2A and 2B is the armthat includes corrosion sensor element 102-1 (note the cross sectionline 2A/B in FIG. 1); however, this is for illustration purposes onlyand each corrosion sensitive resistor arm of the sensor 100 is formedand operates in a similar manner as described herein.

As shown, in FIGS. 2A and 2B, a protective film 202 is formed on the topof corrosion sensor element 102-1. Note that as shown here in thesubject figures, the sensor element 102-1 is formed on top of thenon-conductive substrate 206. The protective film 202 can be formed fromsuch material as Al₂O₃ (aluminum oxide) or Si₃Ni₄ (silicon nitride). Itis to be understood that the protective layer 202 is preferably formedvia the electron-beam fabrication process that is used for the depositedmetal film (sensor elements). The protective film may be also be made ofthe same material as the encapsulation layer 112 (recall that theencapsulation layer encapsulates the reference sensor elements 104 andthe conducting bus 106).

Thus, note that for the corrosion sensor element 102, the protectivefilm coating covers the top of the sensor element (as shown in FIG. 2A),but in the case of the reference sensor elements 104, the protectivefilm (encapsulation layer) covers both the top and the sides of themetal film resistors such that they are fully protected from corrosion.This is also the case for the conducting bus 106 so that it is fullyprotected from corrosion. Note that the bottoms of the sensor elements102 and 104, and the conducting bus 106 are protected from corrosion bythe substrate 206.

More specifically, the protective film 202 shown in FIG. 2A serves toprevent the corrosion sensor element 102-1 from corroding in a spatialdimension other than width w. That is, since the height h of the sensorelement 102-1 is contained by the substrate 206 on one end and theprotective film 202 on the other end, as the sensor element is exposedto contaminants, it corrodes in a single spatial dimension, i.e., widthw. That is what reference numeral 204 denotes, i.e., corroded lateralsides 204. Thus, the two respective sides of the sensor element 102-1corrode an amount denoted by A and B, and this is what Δw represents (Aplus B).

The resistance R of a given metal film (wire) prior to corrosion (inthis case, corrosion sensor element 102-1 in FIG. 2A) is represented as:

$\begin{matrix}{R = {\rho\frac{L}{wh}}} & {{Eq}.\mspace{14mu}(1)}\end{matrix}$where w is the width of the sensor element, h is the height of thesensor element (wh denoting the area of the metal film), L is the lengthof the sensor element, and p is the electrical resistivity (also knownas specific electrical resistance or volume resistivity) of the sensorelement. Electrical resistivity is a measure of how strongly a materialopposes the flow of electric current (measured in ohm meters).

The resistance R_(corr) of a given metal film after or during corrosion(in this case, corrosion sensor element 102-1 in FIG. 2B) is representedas:

$\begin{matrix}{R_{corr} = {\rho\frac{L}{\left( {w - {\Delta\; w}} \right)h}}} & {{Eq}.\mspace{14mu}(2)}\end{matrix}$

where Δw represents A plus B (i.e., the corroded width from both lateralsides of the metal film).

Recall from FIG. 1 that the reference sensor elements 104 areencapsulated via encapsulation layer 112 so as not to be affected bycorrosive contaminants. Thus, the resistance R of each of these metalfilms (wires) will remain substantially constant and thus be representedby Eq. (1). That is, there is no Δw associated with the reference sensorelements 104.

Note that while FIGS. 2A and 2B show corrosion effects for corrosionsensor element 102-1, the same corrosion effects and resulting equationsapply to the other corrosion sensor elements (102-2, 102-3, etc.).

A measurement operation of the corrosion sensor 100 will now beexplained in the context of FIG. 3. A constant voltage V is applied toone end of the sensor 100 (at contact pad 110) and the current passesthrough the connecting bus 106 to each of the variable width arms of thesensor 100. The current passing through each sensor element (eachvariable width arm) is monitored. For example, in one embodiment, thecurrent may be converted to voltage using a transimpedance amplifier(not shown, but generally depicted as monitor(s) in FIG. 3) where theresulting voltage is proportional to the width of the metal films(wires). As corrosion proceeds, each sensor element 102 corrodeslaterally and its resistance increases as per Eq. (2), i.e., as Δwincreases, the denominator of Eq. (2) gets smaller, and the resistanceR_(corr) increases.

Thus, in accordance with the present invention, by measuring the currentflowing through corrosion sensor element 102-1 and measuring the currentflowing through the reference sensor element 104-1, and comparing thetwo measurements, the result will be directly proportional to thecorrosion thickness.

That is, as shown in FIG. 3, the current measured from reference sensorelement 104-1 is:

$\begin{matrix}{I_{10} = {\frac{V}{R_{10}} = {V\frac{w_{1}h}{\rho\; L}}}} & {{Eq}.\mspace{14mu}(3)}\end{matrix}$while the current measured from reference sensor element 104-2 is:

$\begin{matrix}{I_{20} = {\frac{V}{R_{20}} = {V\frac{w_{2}h}{\rho\; L}}}} & {{Eq}.\mspace{14mu}(4)}\end{matrix}$Furthermore, the current measured from corrosion sensor element 102-1is:

$\begin{matrix}{I_{1} = {\frac{V}{R_{1}} = {V\frac{\left( {w_{1} - {\Delta\; w}} \right)h}{\rho\; L}}}} & {{Eq}.\mspace{14mu}(5)}\end{matrix}$while the current measured from corrosion sensor element 102-2 is:

$\begin{matrix}{I_{2} = {\frac{V}{R_{2}} = {V\frac{\left( {w_{2} - {\Delta\; w}} \right)h}{\rho\; L}}}} & {{Eq}.\mspace{14mu}(6)}\end{matrix}$

Thus, at beginning before corrosion effects are present (FIG. 2A), thedifference between, for example, the current from the corrosion sensorelement 102-1 and the reference sensor element 104-1 (I=I₁−I₁₀) is zero.However, as corrosion proceeds (FIG. 2B), the current difference changeslinearly, as the width of the corrosion sensor element 102-1 decreases.The maximum current would be when the corrosion sensor element is fullycorroded. This may be denoted as:

$\begin{matrix}{I = {{I_{1} - I_{10}} = {V\frac{\Delta\;{wh}}{\rho\; L}}}} & {{Eq}.\mspace{14mu}(7)}\end{matrix}$

Advantageously, the current I represents the rate of corrosion for theparticular corrosion sensitive resistor arm being monitored. It is to beappreciated that this same signal measurement and comparison applies toeach pair of reference and corrosion sensor elements.

All elements are connected to detection circuitry (a monitor) and theywill record the corrosion rate as it proceeds for all of the differentwidths metal films. The parallel measurement allows for correlation ofthe corrosion rate from all the different width sensors. The same ratewill be measured by all of the sensors. As expected, the widest sensorwill have the longest operational lifetime.

To enhance the sensitivity of a corrosion sensor detection to be able tomonitor small a corrosion rate, a corrosion sensor can be integrated ina bridge circuit setup as illustrated in FIG. 4.

As shown, the bridge setup 400 comprises three corrosion sensors 402,404 and 406 (although more or less sensors can be employed). Eachcorrosion sensor is comprised of two thin film sensor elements (metalfilms or wires) that are exposed to corrosion (410-1 in sensor 402;410-2 in sensor 404; and 410-3 in sensor 406) and two thin film sensorelements (metal films or wires) that are covered with conformal coating411 (i.e., similar material as encapsulation layer 112 and protectivefilm 202) so as not to be exposed to corrosion (412-1 in sensor 402;412-2 in sensor 404; and 412-3 in sensor 406). The conformal coatedsensor elements are the reference sensor elements while two other sensorelements are exposed to the corrosive environment and their resistancewill change as the width of the thin film decreases. The widths of thesensor elements for each corrosion sensors 402, 404 and 406 (i.e., w1,w2 and w3) progressively increase. This provides the progressive timeperiod corrosion monitoring explained above with respect to thecorrosion sensor in FIG. 3.

At the beginning, all the sensor elements in a given corrosion sensorhave the same value and the bridge circuit 400 is balanced, i.e., thedifferential voltage from the two node points (V01 and V02 for sensor402; V03 and V04 for sensor 404; and V05 and V06 for sensor 406) iszero. However, small changes in the resistance value of the sensorelements not conformally coated will take the bridge circuit out ofbalance. The bridge can be operated under constant voltage V that woulddecrease the current flowing through each corrosion sensor circuit asthe corrosion resistance value increases. The bridge circuit 400advantageously allows a two fold increase in resistance changesensitivity detection.

Advantageously, in this particular embodiment, the bridge circuitarrangement of FIG. 4 is constructed such that all the parts of thesensors (402, 404, and 406) are fabricated from the same metal havingtwo opposite elements of the bridge exposed to the corrosive environmentwhile the two other elements are protected from the corrosive atmosphereby a protective coating. For a corrosion sensor in the bridge circuitarrangement, all four sensor elements are equal in size such that whennone of the corrosion sensors experiences corrosion, the bridgearrangement is in a balanced condition, and when at least one of thecorrosion sensors experiences corrosion, the bridge arrangement is in aunbalanced condition. Since all the elements of the bridge circuit arefrom the same metal deposited on the same substrate, the temperaturevariation that can cause resistance changes are advantageouslyeliminated.

As an example, the detection sensitivity for 120 nm and 240 nm widesensors (which are actually 60 nm and 120 nm, respectively, as thecorrosion proceeds from both sides) is shown in FIG. 5. Both sensorshave sufficient dynamic range so as to be able to detect variation downto about 0.1 nm variation in film width.

Furthermore, in an integrated circuit implementation of the invention,multiple integrated circuit dies are typically formed in a repeatedpattern on a surface of a wafer. Each such die may include a devicecomprising corrosion detection circuitry as described herein, and mayinclude other structures or circuits. Still further, in anotherembodiment, the corrosion detection circuitry could be implemented inmultiple dies and in multiple integrated circuit packages. In any case,the dies are cut or diced from the wafer, then packaged as integratedcircuits. One skilled in the art would know how to dice wafers andpackage dies to produce packaged integrated circuits. Integratedcircuits so manufactured are considered part of this invention. Thus,methods for forming one or more of the components of a corrosion sensordevice as described herein are within the scope of the invention.

Accordingly, as has been illustratively described herein, principles ofthe invention provide a corrosion sensor with a linear response, as thecorrosion proceeds, combined with ultra low corrosion rate sensitivity(e.g., 10 nm/month). The sensor is based on the width reduction of ametal film (wire) exposed to a corrosive environment and its response iscompared to a reference resistor that is shielded from the corrosiveenvironment. Employing different width metal films (wires) the sensorcan be deployed over an extended period of time while maintaining thesame corrosion sensitivity. The sensor can be deployed both formonitoring internal and external environmental conditions in real time.

It will be appreciated and should be understood that the exemplaryembodiments of the invention described above can be implemented in anumber of different fashions. Given the teachings of the inventionprovided herein, one of ordinary skill in the related art will be ableto contemplate other implementations of the invention. Indeed, althoughillustrative embodiments of the present invention have been describedherein with reference to the accompanying drawings, it is to beunderstood that the invention is not limited to those preciseembodiments, and that various other changes and modifications may bemade by one skilled in the art without departing from the scope orspirit of the invention.

What is claimed is:
 1. An apparatus, comprising: a first sensor element;and at least a second sensor element; wherein the first sensor elementand the at least a second sensor element are susceptible to corrosioncaused by an operating environment in which they reside, and wherein theat least a second sensor element has a corrodible spatial dimension thatis progressively larger than a corrodible spatial dimension of the firstsensor element such that the at least a second sensor element providesfor corrosion monitoring for a time period that is progressively largerthan a period of time for corrosion monitoring associated with the firstsensor element.
 2. The apparatus of claim 1, wherein the corrodiblespatial dimension is width.
 3. The apparatus of claim 2, wherein thewidth of the at least a second sensor element is about two times a widthof the first sensor element.
 4. The apparatus of claim 3, furthercomprising at least a third sensor element that is susceptible tocorrosion caused by an operating environment in which the sensorelements reside, and wherein the at least a third sensor element has acorrodible spatial dimension that is progressively larger than thecorrodible spatial dimension of the second sensor element.
 5. Theapparatus of claim 1, wherein a corrosion rate associated with the firstsensor element is equivalent to about half of the corrodible spatialdimension of the first sensor element.